In the last decade, the interest in obtaining multimodal images, mainly PET/CT or SPECT/CT systems, has been increasing, aiming at acquiring structural (anatomic) and molecular medical images simultaneously. These systems open the possibility of obtaining metabolic or molecular images “in vivo” such as for example, by the PET technique. The most commonly used tracer in PET device is FDG (FluorDeoxiGlucose) which accumulates in anatomic regions with a high metabolism. These regions may be directly related to the anatomical or structural images obtained with the CT device.
More recently, the interest in the development of hybrid PET-SPECT/MR systems has increased, where mainly the anatomical information is obtained from MR, with the consequent reduction of exposure to ionizing radiation compared to CT systems. In a hybrid and simultaneous system, data acquisition times are also reduced, allowing more patients to be explored, as well as obtaining until now unique information, with the simultaneity of PET and MR data. Compared to CT systems, MR systems generally provide a greater contrast in soft tissues, more recently also in hard tissues, and a better spatial resolution in anatomical images of 50-100 μm [1], allowing, additionally, providing information on different physiological parameters.
From the technological point of view, the integration of a PET or SPECT device into an MR device is a great challenge. Mainly, there are two important problems that need to be solved when integrating PET-SPECT and MR device into a single device. The first problem is that the behavior of MR device can be affected by the presence of the elements used in the construction of PET-SPECT devices. For example, the detectors or the electronics associated with them may contain some quantities of ferromagnetic materials that can alter the magnetic field generated by MR devices. The second problem is the potential interferences in the electronics of the PET-SPECT device, generated by the RF signal that is produced in the RF coils with which the MR devices are provided.
To avoid interference of the RF signal in the electronics of the PET or SPECT device, a shield constructed with a non-ferromagnetic conductive material can be placed at this RF field. This shield is usually installed in the mechanical part that covers the detector block and does not interfere with the passage of the light produced in the scintillating crystal towards the photodetectors of the PET-SPECT device. Currently, SiPM (Silicon Photomultiplier) or APD (Avalanche Photodiodes) are the most proposed types of photodetectors in the design of hybrid PET/MR systems, since their operation is not affected by magnetic fields.
The problem is that RF shielding can also affect the magnetic field generated by MR device. By using a conductive material in this RF shield, both the gradient field at low frequencies and the RF signals generated by the MR device produce electric currents called Eddy currents on the surface of this shield, which can affect the homogeneity of the magnetic field of the MR device.
There are several solutions to reduce these currents: one is to divide the RF shield into smaller sections, smaller than 20×20 mm [3], or to increase the separation distance between the PET-SPECT device and the uniform magnetic field region of the MR device, what reduces the detection efficiency of the PET-SPECT device.
One possible solution is disclosed in U.S. Pat. No. 7,218,112-B2 which discloses a combined PET/MR system, in which the RF shield consists of a multitude of apertures, and the scintillation crystals of the PET detector are positioned in said apertures in such a way that at least one part of the crystals are located in the area of the reflux field of the RF. A drawback of such a solution is that the manufacturing process of the crystals must be modified to introduce in each of them the deposition of the shielding material. In any case, said solution is very different from that adopted in the present invention.
U.S. Pat. No. 8,823,259-B2 discloses a graphene sheet for the protection of photocathodes such as QE photocathodes—high quantum efficiency—. A graphene monolayer serves as a transparent screen that does not inhibit the passage of photons or electrons, but that isolates the photosensitive film from reactive gases avoiding contamination and extending the life of the photocathodes. The graphene sheet is placed on the photosensitive film in direct contact with it. In another embodiment the photocathode comprises: a film, the graphene sheet having a first and a second surface, a graphene support on a first portion of the first surface of the graphene sheet, and is configured to form a second portion of the first surface of the graphene sheet, said second portion having no graphene support, in such a way that the second portion of the graphene sheet is placed on the photosensitive film and in direct contact with it. Therefore, this shielding system is different from the one of the present invention, wherein it is intended to protect detector modules of medical imaging devices from the RF.
Another patent application, US20130068521-A1, discloses an electromagnetic field shield, in general, and a method for protecting the electromagnetic radiation using graphene on the inside, or the outside, of the source of electromagnetic waves and/or using graphene formed on a substrate, and also discloses an electromagnetic radiation protective material containing graphene, whereas in the present invention it is intended to shield the detector modules with a Faraday cage structure, minimizing Eddy currents and, in addition, allowing the passage of scintillating light, generated in the scintillating crystal, towards the photodetectors.
Patent application WO2011087301—relates to a method for forming a protective barrier of graphene, which has barrier properties against gas and moisture. A single or multiple layer of graphene can be used to protect various types of devices.
Generally, the RF shielding system consists of a layer of a non-ferromagnetic conductive metal (copper, silver or gold) or also of carbon fiber based composites, said layer completely covering the PET-SPECT device, or each of the PET-SPECT device modules individually. The shielding systems present the problem that both the gradient field and the RF field generated by the conventional MR devices generate electric currents called Eddy currents, on the surface of this shield, which can affect the uniformity of the magnetic field in the MR field of vision [4].
The aim of the present invention is the development of radiofrequency (RF) shield based on graphene and on non-ferromagnetic conductive materials to protect the electronics of the detection modules from the RF signal generated by the MR device, allowing the passage of scintillating light when this shield is placed between the scintillating crystal and the photodetectors. In addition, this electromagnetic shield must not shield or modify the magnetic fields generated by the main field or by the MR device gradient system.
In this invention a new type of radio-frequency shielding, of the Faraday cage-type, is presented, specific for hybrid imaging devices with the additional objective of reducing Eddy currents generated in the RF shielding, which may affect the homogeneity of the magnetic field generated by the MR device, as well as the objective of not shielding the main magnetic field and the magnetic fields generated by the gradient system of MR devices. The result is a RF shielding design, based in part on graphene, installed in the PET or SPECT devices, that allows the construction of more compact PET/MR or SPECT/MR hybrid devices, and therefore with better performance.
According to the present invention the graphene is placed just (very close) in front of the photodetector system to allow greater space (the crystal thickness) between the RF coil and the shield. Since the thickness of the graphene deposition is of one or two atoms, the light crosses it in a high percentage, also static or low frequency magnetic fields are crossed, but not the RF.
Throughout the present specification the terms “coating” and “shielding” are used with a completely equivalent meaning. Likewise, the terms “external” and “outer” are used with identical meaning. Also the terms “internal” and “inner” are used with identical meaning. We will refer to the combination of PET and MR, or SPECT and MR, as medical imaging hybrid systems or devices.
The expression “output face of the photons generated in the scintillating crystal” is equivalent to “face toward the photodetector”